As use of solar and wind to supply electric grid demands grows the intermittent nature of these energy sources becomes more and more significant. To offer the obvious case, solar panels don’t work when there is no sun. The most common way this problem is being addressed is by adding battery storage to the grid. It works great but is expensive.
If you can connect electric grids over long distances in an East-West manner you can “extend” the number of hours when solar power is available. In a similar manner, you can interconnect other intermittent sources such as wind to help average out different wind conditions in different geographical areas.
The obvious question is whether such systems are practical. The good news is that some such systems have already been constructed. They work and they are cost effective. An excellent video by Engineering with Rosie talks about the technology, the current successful implementations and the future.